Technical Field
Embodiments of the invention relate generally to power generation systems and, more particularly, to a system and method for supporting a boiler load.
Discussion of Art
Steam boiler plants generally have large furnaces which are commonly constructed of a number of water-cooled tubes welded in side-by-side arrangement to form gas tight tube banks forming the walls of the furnace. Boilers may be supported from the bottom, middle, or top depending on, for example, the particular application and the size of the boiler. Typically, package boilers, pre-engineered oil- and gas-fired boilers, and solid fuel-fired boilers up to about 60 tph can be bottom-supported. In a bottom-support design, a supporting structure is utilized to support the weight of the boiler from below, and expansion of the boiler pressure parts and hot structural parts occurs upward.
Beyond a certain size, however, top-support designs are typically employed. In particular, as boiler size is increased, differential expansion of the pressure parts and hot structural parts and the weight of the boiler increases, making it cheaper to adopt top support. Top-support designs can be likened to a church bell, whereby all pressure parts and other components are suspended from structural members (e.g., girders) of the steam generating plant. In top-support designs, as the furnace approaches operating temperature, the furnace walls expand vertically downward.
In all boilers, pressure excursions within the furnace (i.e., either an increase or a decrease in pressure within the furnace) cause a resultant additional flexing of the tube walls either inwardly or outwardly in a horizontal direction. It has therefore become customary and necessary to provide an arrangement of flanged girder beams, typically referred to as buckstays, that extend around the furnace to provide additional support to the furnace walls and prevent substantial movement of the furnace walls in a horizontal direction as affected by pressure differential.
Typically, these buckstays are disposed in bands around the perimeter of the furnace walls at vertically spaced intervals throughout the height of the furnace. Horizontally, the buckstays on opposite walls of the furnace are interconnected through buckstay ties so that the reactions of one buckstay are resisted by the reactions of the buckstay on the opposing wall so as to counteract the pressure forces acting on the furnace walls. Vertically, it has been customary to provide vertical support members to interconnect each buckstay to its upper and lower neighbors with a connection that permits a sliding action which is required due to relative movement between the furnace tube walls to which each buckstay is connected and the buckstays themselves.
Certain boiler applications require that the bottom of the furnace serve as a storage hopper for bottom ash accumulation. As will be readily appreciated, such accumulation and storage of bottom ash at the bottom of the furnace creates a large live load on the boiler, which contributes to additional difficulties in the design and construction of buckstays and pressure parts.
Existing solutions to account for the weight of accumulated bottom ash have been to beef up the furnace buckstay system and top supporting members (e.g., building framework, pressure part hangers, pressure part support straps, etc.). These methods, however, can costly, difficult to implement, especially where the capacity for substantial bottom ash accumulation is desired. For example, existing methods for providing additional support for top-supported boilers may no longer be adequate for large boilers in which a large amount of bottom ash storage capacity is desired.